<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <!-- 第一种 -->
    <!-- <div class="parent">
        <div class="test">

        </div>
    </div> -->
    <!-- 第二种 -->
  <!-- <div class="parent">
        <div class="test">

        </div>
    </div> -->
    <!-- 第三种 -->
      <div class="parent">
        <div class="test">

        </div>
    </div>
</body>
</html>
<script>

</script>
<style>
    /* 第一种 */
/* .parent{
    border: 1px solid #333;
    width: 100px;
    height: 100px;
    display: flex;
    justify-content: center;
    align-items: center;
    }
.test{
    width: 40px;
    height: 40px;
    background-color: pink;
} */
/* 第二种 */
/* .parent{
    border: 1px solid #333;
    width: 100px;
    height: 100px;
    position: relative;
    }
.test{
    width: 40px;
    height: 40px;
    background-color: pink;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%,-50%);
    
} */
/* 第三种 */
/* .parent{
    border: 1px solid #333;
    width: 100px;
    height: 100px;

    }
.test{
    width: 40px;
    height: 40px;
    background-color: pink;
    margin-top: 50%;
    margin-left: 50%;
    transform: translate(-50%,-50%);
} */
</style>